Medical imaging is a field dominated by high cost systems that are so complex as to require specialized personnel for operation and the services of experienced physicians for image interpretation. Medical ultrasound, which is considered a low cost modality, utilizes imaging systems costing as much as $250K. These systems are operated by sonographers with several years of training or specialized physicians. This high-tech, high-cost approach works very well for critical diagnostic procedures. However it makes ultrasound impractical for many of the routine tasks for which it would be clinically useful.
The block diagram of a conventional phased array ultrasound system 10 is shown in FIG. 1. A piezoelectric transducer array 12 (or using alternative electrical/ultrasound transduction mechanism—e.g. capacitive micro-machined devices), shown on the left, acts as the interface to the body by converting electrical signals to acoustic pulses and vice versa. Image formation begins when the states of the transmit/receive switches (TX/RX switches 14) are altered to connect the transducer elements to individual transmit generators TX 16. The transmit generators 16 output time varying waveforms with delay and amplitude variations selected to produce a desired acoustic beam. Voltages of up to approximately 150 Volts are applied to the transducer elements. Once transmission is complete, the state of the TX/RX switch 14 is changed to connect the receive circuitry to each element. Incoming voltage echoes are amplified by preamplifiers (preamp 18) and Time Gain Control (TGC 20) circuits to compensate for signal losses associated with diffraction and attenuation. Next, sample and hold circuits (S/H 22) and analog to digital converters (A/D 24) digitize the signals. Finally, the signals are dynamically delayed and summed within one or more custom integrated circuits 26 to yield a single focused Radio Frequency (RF) echo line. This signal forms the basis of one image line.
While conventional beamforming approaches produce high quality images, they also impose significant restrictions on the use of ultrasound. The 40 Msample/s S/H and A/D circuits employed by these systems, and the high data rates they engender, result in high system cost and complexity. A modern state-of-the-art imaging system may cost as much as $250,000 and require weeks or months of user training to produce the highest quality images. Furthermore, while the transducers used by these systems are typically only a few centimeters on a side, the electronics required to form images resides in a box with dimensions on the order of 2′×3′×4′. Thus, while ultrasound systems are certainly portable, they are far from the scale that would allow each clinician to carry one in a pocket.
The applicability of conventional ultrasound is further limited by the typical image format used. Images are produced in what is commonly referred to as a B-Mode format, representing a tomographic slice through the body perpendicular to the skin surface. This image format is non-intuitive and the act of mentally registering the B-Mode image to the patient's anatomy requires significant experience.
Significant reductions in system cost and complexity have occurred over the last five years. Some of the more notable advances have been demonstrated by Sonosite. Its most recent product, which sells for approximately $12,000, produces B-Mode and color flow images using a hand-held system. This system produces good quality images and will certainly broaden the applications for ultrasound in medicine. Unfortunately, the Sonosite architecture and strategy does not appear to be capable of extension to real-time 3D imaging. Furthermore, the B-Mode image format produced by the Sonosite system and all other conventional systems is not intuitive to most first-time ultrasound users. Novice users often have difficulty mapping the image displayed on the screen to the tissue lying beneath the transducer. This most likely results from the distance (a few feet) and orientation differences between the target and image.
Ultrasonic imaging has the potential to be a common component of nearly every medical examination and procedure. But the complexity and expense of the existing ultrasound systems are an impediment to its widespread use. Consequently, an improvement is desired.